Although power supply circuits are commonly regulated, regulation arrangements are typically not able to provide adequate protection against overload or short circuit conditions. Therefore, it is common to design such circuits to include protection arrangements which may be responsive to output (or input) current sensing. In addition, many current sharing approaches employed to provide multiple converters in parallel require output current sensing.
A challenge in designing such current sensing arrangements involves the selection of suitable current sense elements. For example, a current sense resistor can provide excellent accuracy, and is simple and inexpensive to use. However, current sense resistors dissipate power and reduce the efficiency of the power converter circuit. This is particularly important in the design of modern low voltage output/high current output applications. A second example, a current sense transformer, does not affect the efficiency of the power converter, but can deliver only AC information, is complicated to use and is costly.
It is common to place an inductor in series with the output of a power supply to provide filtering. Power inductors are known to have parasitic (or inherent) resistance, and therefore can be represented by an equivalent circuit of a series combination of an ideal inductor and a resistor. When direct current (DC) flows through the inductor (or a current having a DC component), a DC voltage drop is imposed across the inductor, which voltage is a product of the magnitude of the DC (component of the) current and the parasitic resistance of the inductor. The DC voltage drop across the inductor can then be sensed to provide information about current flowing through the inductor. Since such an inductor may already be present in the circuit, there would normally not an additional loss of efficiency in using the inductor for this purpose.
U.S. Pat. No. 5,465,201 (the '201 patent), issued to Cohen, discloses an overload protection arrangement for switch mode power converters. U.S. Pat. No. 5,465,201 is incorporated herein by reference. The protection arrangement includes an output stage in which an inductor is connected in series with the load, and provides an output current to the load. The inductor has an inherent resistance, and the output current is sensed by measuring a DC voltage. The DC voltage is compared with a reference voltage to provide an error signal which can modulate either the frequency of operation of the converter, or its duty cycle, or both. Furthermore, as the temperature of the output inductor rises, its inherent resistance rises also. The rise in inherent resistance of the output inductor leads to a further change of the DC voltage developed across the output inductor.
FIG. 1 is the circuit of FIG. 4 from the '201 patent. This circuit is described beginning at column 4, line 27 of the '201 patent. In the circuit of FIG. 4 of the '201 patent, it can be seen that there is a connection to a first terminal of the inductor 116 (including the inherent resistance 118) which is opposite the connection to the load 122, i.e. at the junction of the inductor 116, the anode of the diode 114, and the secondary winding 106. However, the reference for the measurement of the voltage across the inductor 116 is the ground connection to the voltage reference V.sub.REF1 128, and the only reference to the second terminal of the inductor 116 (including the inherent resistance 118) is through the load 122 and capacitor 120, back through the diode 112, the transformer 102, and V.sub.in 100 to ground. However, since transformers do not pass DC, the DC reference to ground is lost. Hence, the DC voltage measured by the RC filter consisting of the resistor 124 and capacitor 126 is not measured directly across the series inductor 116. It can be seen that in each of the circuits disclosed in the '201 patent, the voltage measured by the sensing circuit is likewise not the DC voltage directly across a series inductor.
Therefore, there is a need for a suitable means of sensing the output current in a switch mode power converter in order to provide protection arrangements which are responsive to overload or short circuit conditions.